The present invention generally relates to a heating element, and more particularly to a thermal inkjet printhead that is suitable for use on a print cartridge. This invention further relates to a method of making the heating element.
Substantial developments have been made in the field of electronic printing technology. A wide variety of highly efficient printing systems currently exist which are capable of dispensing ink in a rapid and accurate manner. Thermal inkjet systems are especially important in this regard. Printing units using thermal inkjet technology basically involve an apparatus which includes at least one ink reservoir in fluid communication with a substrate (preferably made of silicon [Si] and/or other comparable materials) having a plurality of thin-film heating elements or resistors in firing chambers thereon. The substrate and resistors are maintained within a structure that is conventionally referred to as a “printhead”. Selective activation of the resistors causes thermal excitation of the ink materials stored inside the firing chambers and expulsion thereof from the printhead. Representative thermal inkjet systems are discussed in U.S. Pat. No. 6,213,587, Whitman, entitled “Ink Jet Printhead Having Improved Reliability”; and U.S. Pat. No. 6,513,913, Schulte et al., entitled “Heating Element of a Printhead Having Conductive Layer Between Resistive layers.”
The operating efficiency of the printhead, with particular reference to the resistors that are used to expel ink on-demand during printhead operation, is an important consideration in the design of a printhead. The term “operating efficiency” shall herein collectively encompass a number of different items including but not limited to internal temperature levels, thermal uniformity of the resistors which affects ink expulsion volume or drop weight, and the like.
The chemical and physical characteristics of the resistors and interconnection components associated therewith which are selected for use in a thermal inkjet printhead will directly influence the overall operating efficiency of the printhead. The terms “interconnection components” or “interconnection structures” as employed herein generally involve the conductive traces and related elements which electrically connect the resistors to a printing control circuitry of the system.
Known printheads include a conductive layer above a resistive layer that defines the resistors. The conductive layer has traces that have respective sloped or beveled sidewalls in direct proximity with the resistors. Such sloped or beveled sidewalls have surfaces that allow more effective deposition of one or more passivation layers thereon that are normally used to protect the resistors and adjacent components from corrosion. Therefore printhead designs have steered clear of vertical sidewalls because it is difficult to deposit the passivation layers on the surfaces of vertical sidewalls. Moreover, these vertical sidewalls form sharp corners in the printhead that are known to trap a barrier material that is used to form the ink or firing chambers above the resistors. Trapped barrier material in the firing chambers acts as a heat insulating layer that prevents heat generated by the resistors from being effectively dissipated into fluid in the respective firing chambers. This heat insulating barrier material causes heat to build up within the printhead (with particular reference to the substrate or “die” on which the printhead components are positioned), thereby affecting the printhead reliability/longevity levels. Printheads with sloped sidewall design therefore partially overcomes such a problem.
However, printheads having conductive traces with sloped sidewalls suffer from a disadvantage. The slope metal etching (SME) processes, such as wet chemical etching and dry etching, used for creating the sloped surfaces is not precisely controllable. In other words, the amount of conductive material removed from a conductive layer to form two spaced apart conductive traces flanking a resistor cannot be precisely determined. Accordingly, the distance between the two conductive traces that defines the “length” or “boundary” of a resistor cannot be precisely defined. Such a process when used to fabricate a printhead may produce inaccurately sized resistors which when used may result in inaccurate drop weights. In a worse case, the resistors of a printhead may not just be individually inaccurately sized, they may be of different sizes in the printhead. Consequently, heat generated by these resistors and thus drop weights from the various ink chambers formed over the resistors may not be uniform across the firing chambers in the printhead. Such non-uniformity in the drop weights may be a barrier to the design of higher-resolution printheads.
Thus, it is desirable to have an inkjet printhead having heating elements wherein the dimensions of resistors therein are more precisely controlled so as to achieve uniformity of drop weights throughout the heating elements.